refactor(core): D.5.3/B.2 — move ShortcutStore to Core.Items (decouple Load from the wire type)

Matches the ClientObjectTable model-placement convention; Load now takes (slot,objGuid) pairs so
the store has no Core.Net dependency. + self-drop wire-count assert + comment fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/Layout/ToolbarControllerTests.cs

@@ -581,6 +581,8 @@ public class ToolbarControllerTests
 
         Assert.Equal(0x5001u, slots[Row1[3]].Cell.ItemId);   // re-added to source (net no-op)
         Assert.Contains((3u, 0x5001u), adds);                // AddShortcut(3, A) sent
+        Assert.Single(removes);   // exactly RemoveShortcut(3) [from the lift]
+        Assert.Single(adds);      // exactly AddShortcut(3, A) [the re-place]
     }
 
     [Fact]